Grade-mounted light fixtures perform a variety of desirable functions, such as illuminating the facades and exteriors of buildings, creating aesthetic light and shadow effects in a range of architectural and landscape settings and providing safety and security lighting around commercial and industrial buildings. Such light fixtures are referred to as grade-mounted because, they are installed into recesses in the landscapes adjacent the buildings so that the uppermost surface of the light fixture, which is typically a transparent cover overlying the lamp, is generally flush with the landscape surface. It can be appreciated that such recessed light fixtures are ideal in environments in which lighting is desired without the unsightliness or disruptive lines of an above-grade light fixture.
In view of their installation into recesses in the landscape, recessed light fixtures require special consideration in their design and construction. For example, the light fixture must be as water tight as possible to avoid the corrosive effect of soil, plants, concrete or other material having an appreciable moisture content. Protection against water penetration is all the more important since visual inspection of such recessed light fixtures usually necessitates withdrawal of the fixture from its recess, a time-consuming and labor-intensive task. Additionally, the very low profile of the recessed light fixtures makes it difficult to successfully adapt these fixtures with desirable modifications such as, for example, photosensitive devices which detect changes in the ambient light conditions and automatically activate and deactivate the lamp without the need for operator intervention. In fact, a number of commercially available photosensitive devices have proven to be impractical or unreliable in their application to the unique environment of recessed light fixtures.
Conventional photosensitive devices generally comprise a switch having a light sensitive diode which generates electrical signals in response to the intensity of light incident upon its detecting means and a light transmitting channel or plug which channels outside light to the photosensitive switch.
One commercially available button type photocontrol device has a sensor cell mounted on a rectangular plastic housing, an arrangement much like a dish on a table top. The sensor cell faces towards the sky, while the bottom of the box rests on the fixture housing. In another commercially available photosensitive device, the housing has an integral threaded, hollow stem which can be threaded directly into a threaded aperture of the fixture housing. A third type of commercially available photosensitive device has a cylindrical housing with a threaded hollow stem for engagement with the aperture of the fixture housing.
However, as can be appreciated, these outer mounted photocontrol devices are ill suited for recessed light fixtures. The flat surface of the sensor cells which receive the ambient light can easily be blocked by dirt, dust, plant debris or other opaque materials which collect thereon. With the sensor cell partially or completely blocked by the accumulated materials, only a significantly reduced amount of ambient light passes through the sensor cell to fall onto and activate the photosensitive diode in the switch. Eventually, when the amount of ambient light falling through to the switch is beneath a preset threshold, the switch responds as if dark conditions prevail outside the light fixture and thus acts to turn on the lamp. As can be seen, such "false reading" by the photocontrol device can waste energy nd unnecessarily reduce the operating life of the lamp and the other light fixture components by turning on the lamp when diminished or dark light conditions do not, in fact, exist.
Additionally, in recessed light fixture applications, it is desirable that the switch be beneath the exterior surface of the landscape grade so the smooth, grade level appearance of the fixture is maintained. In addition to marring the aesthetic appearance of the grade-mounted light fixture, these known photosensitive devices can interfere with pedestrians walking over the outer surface of the fixture or with lawn mowers or other landscape maintenance machines which pass over the light fixture.
The interference problems caused by the excessive height of the known photocontrol devices can be reduced if the switch component of the photosensitive devices is disposed beneath the outer surface of the fixture while the light channeling plug is the sole component which projects above the outer surface of the light fixture. It is known to dispose the switch components of photosensitive devices within the housing of a light fixture while positioning the light collecting plug on the exterior of the housing. For example, see U.S. Pat. No. 3,264,466 to Bacon, Jr., U.S. Pat. No. 3,274,392 to Harling and U.S. Pat. No. 3,543,099 to Turner. These patents are directed to photosensitive devices disposed within street lamps or other light fixtures positioned some distance above ground. However, while these patents disclose light collecting plugs which prevent, to some degree, the accumulation of dirt or other opaque material on their outer surface -- see, for example, the dome shaped collectors of Bacon and Turner and the conical light collector of Harling -- these patents offer little guidance to the successful adaptation of a photosensitive device to a grade mounted or recessed light fixture. For example, the conical light collector of Harling is angled so that a desirable amount of light rays are reflected by the reflective surface of the collector onto the underlying photosensitive cell. Thus, the height to which the conical collector projects above the light fixture housing is not a design constraint as it is with received light fixtures.